The invention relates to a sealing ring assembly and an improved method for mounting a sealing ring into an electrochemical cell so as to improve the repeatability of electrochemical capacitance voltage (ECV) measurements.
Semiconductor devices are made by sandwiching layers of material of different electrical and/or optical properties together. The layers are formed by epitaxial growth on or ion implantation or diffusion into a substrate wafer. Correct device operation necessitates close control of layer properties including carrier concentration and thickness. U.S. Pat. Nos. 4,028,207 & 4,168,212 describe the ECV profiling method, which is used for determining the carrier concentration as a function of depth into the layer and is therefore ideally suited to measuring these parameters.
ECV profiling makes use of the diode structure formed when a conducting liquid (electrolyte) is placed in contact with a semiconductor. The capacitance of the junction, in the reverse bias region, is determined by the magnitude of the applied bias and the carrier concentration vs. depth profile. By measuring this capacitance as a function of bias the carrier concentration depth profile can be determined. In this mode of operation the ECV profiler is similar to tools, which use metallisation or mercury in place of an electrolyte to form a diode structure. However, such tools are usually restricted to shallow depth profiles due to reverse bias breakdown of the semiconductor-metal junction. By using an electrolyte this limitation is overcome. The electrolyte is used to electrochemically etch into the sample, increasing the depth profiled without increasing the measurement bias. This makes ECV profiling a very powerful method for characterizing multi-layer structures.
ECV profiling is carried out using electrochemical cells of designs which will be familiar to the skilled person. The cell provides a conducting electrolyte reservoir for the profiling process and includes suitable reference and counter electrodes. Electrolyte is caused to contact with a semiconductor sample under test, which becomes the working electrode of the cell. Usually a chemically resistant plastic ring, referred to as a sealing ring, defines the area of contact between the electrolyte and the semiconductor sample. Any damage to the sealing edge of the ring has a big effect on the measurements. In extreme cases the seal will leak and no measurement is possible. Usually the sealing ring is pressed into a counter bore in the electrochemical cell body. This action involves applying pressure close to the seal and there is always the possibility of contacting the seal and damaging it.
Accurate measurements depend on knowing the precise area of contact between the test sample and the electrolyte and ensuring that it remains stable throughout the measurement process. Some seepage under the lip of the ring is inevitable and this will give rise to errors in the analysis, but the design of the ring is such that this is minimized and stable for any given electrolyte.
The sealing ring is made by injection moulding and its small size makes it difficult to handle and avoid accidental damage to the sealing edge. Ideally the sealing edge should make a knife edge contact with the sample although some compliance is essential to allow for slight misalignment or non planer samples. However to avoid excessive deformation, the edge has to be sufficiently rigid and this is achieved by employing a cone like structure with the material progressively getting thinner towards the sealing edge. The bore of the cone is also tapered to ensure the surface of the sample can be uniformly illuminated through the electrolyte.
Gas bubbles can affect the measurement during profiling. Gas bubbles arise due to trapped air when the electrochemical cell is initially filled, or during electrolyte circulation or are a result of the electrochemical reactions used to etch the sample. Gas bubbles lead to non-planer etching which changes the measurement area and can seriously affect measurement quality when multi-layer structures or structures with changing carrier concentration are being profiled. The bubbles can be removed by circulating the electrolyte in the cell, but the shape of the sealing ring makes this difficult, giving rise to dead spots where there is little or no flow allowing bubbles to accumulate.
In many prior art systems the semiconductor sample is mounted vertically. This helps prevent reaction products dropping back onto the surface of the sample and makes it easier to drain the electrochemical cell at the end of the measurement. However in this orientation gas bubbles are more prone to collect in the ring and affect the ECV profiling measurement.
Electrochemically etching occurs when the sample is the anode (biased positively with respect to a counter electrode). For p-type material this is the forward bias condition and electrochemical etching readily takes place. For n-type material this is the reverse bias condition and electrochemical etching only occurs when the sample is illuminated with light of energy above the band-gap of the material. The requirement to illuminate the sample surface for n-type materials means that any form of pumping system used to circulate the electrolyte in the ring, must not obstruct the illumination system.
Several schemes for removing bubbles and reaction products have been described. German Pat. Nos. DE3103611 discloses a device, which uses ultra-sound to perform this task. A modified sealing ring design is shown in “I. Mayes—Electrochemical C-V Profiling of Silicon”, ECS Symposium on Diagnostic Techniques for Semiconductor Materials and Devices, 1992, Vol 92-2, p249-260”. The sealing ring uses three tangential jets spaced 120 degrees apart to generate swirl in the electrolyte and sweep bubbles and other reaction products away from the surface of the sample, which in this case is mounted horizontally.
However this design is not suitable for vertically mounted samples. Gas bubbles tend to collect behind the jets and are not removed unless the electrolyte flow rate is very high. In practice high flow rates are difficult to achieve. Peristaltic and diaphragm pumps tend to produce undesirable pressure fluctuations. A magnetically coupled centrifugal pump meets the requirement for the pump to be chemically inert, non-shedding and be capable of being easily drained. Such pumps are prone to cavitation at high speeds and their use in this application is therefore restricted to reduced flow rates.